Many disposable absorbent articles use fluff pulp as the absorbent core. Such cores are generally soft, flexible and absorbent but tend to be bulky and thick and have poor wicking properties.
An absorbent structure that has poor wicking properties may increase the likelihood of failure of the absorbent product to hold and contain body fluids. Body fluids will be localized to a certain area of a poorly wicking absorbent core and cause saturation in such area whereby excess fluid may overflow through an external surface of the absorbent product. This overflow may contact the user's garment and cause stains or contact the user's body and cause wet discomfort or rash.
It is therefore desirable to provide an absorbent core for disposable absorbent articles which can wick away body fluids from the point of contact with the absorbent core and spread it throughout the absorbent core to more efficiently utilize the entire surface area of the absorbent core. The improved wicking properties of such an absorbent core provides the capacity for fluids to travel by capillary action throughout the surface area of the absorbent core and thus permit the use of thinner cores, since more absorbent surface area can be made available for absorbing body fluids by such wicking action. Thinner structures of absorbent cores may prove to be more comfortable for the user and less unsightly or obvious when worn under clothes.
Absorbent cores with excellent wicking properties comprising peat moss and wood pulp composite materials are described, for example, in U.S. Pat. Nos. 4,170,515; 4,226,237; 4,215,692; 4,507,122; 4,676,871; and 4,473,440. In accordance with the teachings of these patents, an absorbent structure comprising peat moss as a primary absorbent component is formed as a board by air or wet laying of fibers and calendaring the board to obtain a relatively thin, i.e. from about 0.01 to 0.10 inch thick, relatively dense, i.e. from about 0.2 to 1.0 gram/cm.sup.3 sheet-like structure. Such absorbent peat moss boards may be processed to increase the flexibility thereof by subjecting such boards to perf-embossing as described in U.S. Pat. No. 4,596,567 or microcorrugating as described in U.S. Pat. No. 4,605,402.
The peat moss boards thus formed have a large proportion of extremely tiny pores and capillaries which give them the ability to absorb and retain an enormous capacity of fluid. The peat moss pores swell as they absorb fluid, however, this swelling does not cause a loss of capacity for absorbing fluid. Rather, the swelling contributes to the ability of the board to retain fluid while generally maintaining the structural integrity of the absorbent structure in use.
The wicking properties of the above-described peat moss boards provide the ability for the boards to be highly absorbent and thin. The flexibility of peat moss board may be improved by perf-embossing and/or microcorrugating as described above.
While peat moss boards make excellent absorbent and wicking cores for disposable absorbent articles, there are limitations to their production and use. Peat moss board may not be readily available particularly in areas which lack the critical raw material, i.e. peat moss or sphagnum moss of desirable age, structure and moisture content. Peat moss board also is relatively dark in color and may not be aesthetically acceptable for use in all absorbent products. It is, therefore, desirable to provide a thin, absorbent and wicking core for disposable absorbent articles which may be substituted for peat moss boards.
Attempts to utilize other cellulosic pulp boards such as kraft wood pulp boards as absorbent cores have not been successful because they tend not to have as much absorbent capacity as peat moss composite boards but more importantly cannot be sufficiently softened for their intended use. While such kraft wood pulp board's flexibility and other characteristics may be improved by perf-embossing or microcorrugating techniques, such products still do not provide a desirable combination of absorption capacity and fluid penetration, wicking rates and most importantly a sufficient degree of flexibility to be useful in disposable absorbent products, particularly, sanitary napkins.
It is, therefore, an object of the present invention to provide a cellulosic pulp board which does not utilize peat moss in its structure but has sufficient absorption capacity, wicking characteristics as well as advantageously short fluid penetration time and possessing optimal flexibility for use in disposable absorbent articles, particularly sanitary napkins. Optimal flexibility of such products requires that the product be comfortably soft and flexible to the wearer but stiff and strong enough to substantially retain its original shape in use or after wetting.